[p5.strands] Detect references to outside variables and give helpful feedback

Increasing access

There are a number of subtle ways you can use variables in p5.strands that give no errors but subtly don't work. We can improve the experience for users to give them next steps for debugging.

Most appropriate sub-area of p5.js?

• Accessibility
• Color
• Core/Environment/Rendering
• Data
• DOM
• Events
• Image
• IO
• Math
• Typography
• Utilities
• WebGL
• Build process
• Unit testing
• Internationalization
• Friendly errors
• Other (specify if possible)

Feature enhancement details

@ksen0 created this example and noted that if you were to use if (mouseX/windowWidth < 0.3) { instead of if (value < 0.3) { then it stops working, but without any errors.

let testShader;
let pixelate;
let state_;


async function setup() {
  state_ = 0;
  createCanvas(windowWidth, windowHeight, WEBGL);
  testShader = baseMaterialShader().modify(() => {
    const value = uniformFloat(() => mouseX/windowWidth);
    const state = uniformFloat(() => state_);

    let time = uniformFloat(() => frameCount/5);
    getPixelInputs((inputs, canvasContent) => {
      let color = inputs.color;
      if (value < 0.3) {
        color = vec3(1.0, state*0.1, 0);
      } else if (value < 0.8) {
        color = vec3(state*0.1, 1, state*0.1);
      } else {
        color = vec3(state*0.1, 0, 1);
      }
      inputs.color = [color, 1.0];
      return inputs;
    });
  });
}

function draw() {
  background(0);
  shader(testShader);
  testShader.setUniform('value', mouseX/width)
  sphere(100);
}

function mousePressed(){
  state_ += 1;
  console.log(state_)
}

When something isn't a uniform, and isn't a value we specifically handle (see https://github.com/processing/p5.js/issues/8171; in the future we'll make mouseX and windowWidth available), it falls into one of two cases:

• It's a dynamic value, and should be turned into a uniform, e.g.: const myValue = uniformFloat() + myShader.setUniform('myValue', something) after applying the shader

• It's a static value, but it's a local variable that the strands callback can't access, and needs to be passed into an object as the second parameter of the modify function, e.g.:

  let myVal = 10
  const myShader = baseMaterialShader().modify(() => {
    getWorldInputs((inputs) => {
      inputs.position.x += myVal
      return inputs
    })
  }, { myVal }) // This is how to make a local variable accessible in strands
  

Maybe that second case isn't common enough to tell people about, so possibly just the first case is fine. Ideally we can detect usages of variables that aren't declared within the p5.strands callback, see if they're a known p5 global, and if not, output an example of making a uniform out of it?